1. Field of the invention
The present invention concerns a method of fabricating electric resistance welding sleeves, a device for implementing it and preferred applications of the sleeve produced in this way.
2. Description of the prior art
Electric resistance welding sleeves are used in particular for jointing plastics material tubular elements such as gas or water distribution pipes usually made from polyethylene.
The electric resistance welding technique consists in using a heating wire embedded in a connecting part and forming an electrical resistance to heat the contacting surfaces of the parts to the temperature at which the materials weld.
The usual technique employed for forming such connecting parts (or connectors) is overmolding by injection molding. The heating wire is placed in a spool which is overmolded to form the connecting part. The wire is therefore relatively near the inside surface of the element. To achieve good quality welding it is desirable for the wire to be as near as possible to this inside contact surface.
One way to fabricate an electric resistance welding sleeve is known from French patent application FR-B-2.221.679 (STURM VON ROLL AG), by forming a groove in the outside surface of a plastics material sleeve and winding the resistance wire under tension into the resulting groove on the sleeve. When finished, the sleeve is expanded by means of a spreader mandrel and this subjects the heating wire to an additional longitudinal tension stress. The sleeve is then surrounded by an outer sleeve element in an injection mold. The major disadvantage of a method of this kind is the risk of displacement and expansion of the heating wire before and during overmolding.
Before it is overmolded the wire is held in place on the surface of the spool only by the tension in it. This tension may not be correct, for example because of an operator error during winding or during storage because of a variation in the diameter of the sleeve due to ambient atmospheric conditions, in particular excessive relative humidity.
It is also difficult to regulate the tension to be applied to the wire as it is wound onto the sleeve. The wire must be firmly tensioned so that it is securely accommodated in the bottom of the helical groove. However, the wire must not be overtensioned because in this case its diameter is reduced and the final resistivity obtained is higher than that required, which has disadvantages at welding time.
The wire may also be displaced during overmolding by the forces due to the viscosity of the overmolded plastics material.
Any such displacements lead to non-homogeneous dispersion of heat, compromising the quality of the weld. There is also the risk of air pockets being created in the connecting part, which are also detrimental to the weld.
What is more, in a method of this kind there is obviously a minimum distance between the wire and the inside surface of the sleeve that cannot be eliminated and it is therefore impossible to have the wire tangential to the inside surface of the sleeve, among other things.
Finally, with sleeves of this kind storage is a problem because the wire and the groove in the sleeve are exposed to the ordinary atmosphere of the workshop so that contamination by polluted air and soiling by dust or the like leads to the necessity to clean them before carrying out the overmolding.
European patent EP-A-303 909 (GLYNWED TUBES AND FITTINGS LIMITED) proposes a method of fabricating an electric resistance welding connector in which a heated wire is wound onto a sleeve. The temperature of the wire softens the plastics material near the wire. A pressure roller transmits to the wire sufficient pressure to embed it in the plastics material, so that the wire is eventually entirely covered with plastics material. The connector is then obtained by overmolding this sleeve in the conventional way.
This method has various disadvantages.
Guiding the wire onto the surface of the sleeve in a helix can only be achieved by translation of the sleeve or of the heating head during rotation of the sleeve. An incorrect wire tension or incorrect adjustment of the tooling can lead to unwanted variation in the pitch of the helix which is detrimental to the homogeneity of the weld and creates a risk of short-circuiting.
Also, the heating wire is installed near the outer surface of the sleeve and may even be flush with it. As explained above, it is at the inside surface of the sleeve that the plastics material of the sleeve and the tubular elements to be jointed must be heated to achieve the optimum weld.
Finally, there is some uncertainty as to the reliability of this method. The pressure roller smooths the plastics material over the wire to only a limited degree and irregularities in the anchoring of the wire may result, possibly even total absence of anchoring at some locations on the surface of the sleeve.
Generally speaking, the methods described in these two prior art documents systematically require overmolding of the sleeve to produce the electric resistance welding connector.
In both cases if there were no overmolding the heating of the wire would destroy the sleeve. This is because of the absence of any continuous external structure in the document FR-A-2 221 679 and because of the reduced thickness of the sleeve and the external disposition of the wire in the document EP-A-0303 909.
European patent application EP-A-0.086.359 proposes to form a helical groove in the inside cylindrical surface of a plastics material sleeve fabricated beforehand, for example by molding. The groove is formed in the cylindrical inside surface by a rotating cutting head fitted with a cutter.
The heating wire is then introduced into the groove formed in this way and freely placed in the bottom of the groove, which is finally partly closed by pressing the surrounding plastics material.
In this cited document the radial position of the wire relative to the cylindrical inside surface is not specified and it is possible that the wire is not held totally against the bottom of the groove. Also, the method described is such that part of the external surface of the wire is not covered by any coating material which makes it impossible to produce a sleeve in which the heating wire is flush with the cylindrical inside surface. Storing a sleeve produced in this way poses problems because the wire remains exposed to the ordinary atmosphere of the shop and contamination by polluted air may make it necessary to clean the sleeve before it is used for welding.
The fitting of the heating wire into an internal groove by smoothing the plastics material so as to surround it only partially prevents effective control over the anchoring of the wire into the plastics material during winding and at the end of the smoothing operation.
An object of the present invention is to remedy these disadvantages and to fabricate an electric resistance welding sleeve that can be used on its own, without overmolding, for example as an electric resistance welding sleeve for routine use, in particular for welding water pipes, or, after overmolding, as an electric resistance welding connector, in particular for gas pipes.